1. Technical Field
The present invention relates to a substrate cleaning apparatus comprising a cleaning member, in which the cleaning member is abutted against a substrate that requires a high degree of cleanliness, such as a semiconductor substrate, a glass substrate, a liquid crystal panel, etc., to thereby effect scrub cleaning of the substrate. More specifically, the present invention relates to a method for determining the timing of the replacement of a cleaning member used for a substrate cleaning apparatus, in which the timing of the replacement of a cleaning member is determined, based on a surface condition of the cleaning member, to thereby maintain a clean condition of a cleaning member. The present invention also relates to a substrate cleaning apparatus.
2. Technical Background
As a conventional method for cleaning a surface of a substrate, such as a semiconductor substrate, scrub cleaning is conducted by rubbing a cleaning member, such as a brush or a sponge, against a surface of the substrate, while feeding pure water onto the substrate surface. Scrub cleaning is highly effective in removing contaminants deposited on the substrate. However, there is a disadvantage that, since the cleaning member is in direct contact with the substrate during cleaning, the cleaning member itself is contaminated as a result of use over a long period of time, and its ability to clean the substrate becomes low. Further, as an amount of contamination of the cleaning member increases, not only the substrate cleaning ability of the cleaning member becomes low, but also further contamination of a substrate occurs due to contaminants deposited on the cleaning member, and thus the effect of cleaning cannot be obtained.
To obviate these disadvantages, various methods have been proposed, such as a method of applying ultrasonic vibrations to the cleaning member while feeding a cleaning liquid to the cleaning member (JP-A-5-317783), a method of cleaning a substrate with a cleaning brush in a cleaning liquid to which ultrasonic vibrations are applied (JP-A-6-5577), and a method of rubbing a cleaning member and an abutting member against each other in a cleaning liquid to which ultrasonic waves are applied (JP-A-10-109074). These methods are effective in removing contaminants in a relatively superficial layer of the cleaning member. These methods are unsatisfactory, however, from the viewpoint of removing contaminants which have penetrated into an interior of the cleaning member. As a countermeasure, it has been proposed to suppress contamination of the interior of the cleaning member by supplying a cleaning liquid from a cleaning liquid supply portion provided in the interior of the cleaning member. In this method, however, it is difficult to clean the interior of the cleaning member at a position remote from the cleaning liquid supply portion.
Deterioration of a material of the cleaning member is also a factor of contamination of a substrate caused by the cleaning member. Due to an operation of abutting and rubbing the cleaning member against a substrate, an amount of wear of a surface of the cleaning member gradually increases. A resin such as polyvinyl alcohol (PVA) is widely used as a material for a cleaning member. It is molded in a porous form, and comprises an exterior layer formed in the vicinity of a surface in contact with a mold during molding, and an interior layer formed inside the exterior layer. The exterior layer formed in the vicinity of an exterior surface of the cleaning member (a thin superficial layer) consists of a hard layer having a thickness of about several μm to about 10 μm, and contains a small number of pores having a diameter as small as several to several tens of μm. The hard exterior layer gradually wears during cleaning, due to a repeated operation of rubbing the surface of the cleaning member against the substrate. Finally, the interior layer formed inside the exterior layer, which comprises a soft resin material, is exposed and brought into contact with the substrate to be cleaned. The pores formed in the soft interior layer have a diameter as large as 10 μm to 200 μm. Since the interior layer is liable to wear as compared to the exterior layer, a number of scraps are generated due to wear when the interior layer is rubbed against the substrate, and cause a so-called particle contamination of the substrate.
By repeatedly rubbing the cleaning member against the substrate, not only a surface of the cleaning member wears, but also contaminants gradually penetrate into the interior of the cleaning member and are accumulated in the cleaning member. When such a cleaning member is used, a rate of removal of contaminants from the substrate lowers, and further contamination of the substrate caused by the cleaning member occurs. To obviate these disadvantages, a cleaning member should be sequentially replaced with a new cleaning member in a clean condition. Therefore, in a conventional technique, the timing of replacement of a cleaning member is determined by predicting, from results of experiments, an appropriate number of substrates to be treated. In fact, however, the effective number of substrates to be treated varies, depending on the condition of the substrate and the conditions of cleaning. There is also a method for determining the timing of replacement of a cleaning member by performing sampling inspection of a substrate, or cleaning and evaluating a substrate prepared for evaluation of contamination during the time between substrate-cleaning operations. In this case, if inspection of a substrate is conducted at short-time intervals, replacement can be conducted at an early stage of contamination of the cleaning member, and a high degree of cleanliness of a substrate can be maintained. However, this method is costly and time-consuming.
If the inspection is conducted at distant intervals, for example, if a substrate for evaluation is supplied at distant intervals, a number of substrates may already be treated by using a contaminated or worn cleaning member at the time of detection of contamination or wear of the cleaning member. This leads to a situation such that substrates after the cleaning operation are contaminated. There is a method for determining contamination of a cleaning member by monitoring a contaminated condition of a waste liquid discharged during a substrate-cleaning operation. In this case, however, contaminants removed from the substrate itself are also contained in the waste liquid, and it is difficult to determine whether a source of contaminants is a substrate or a cleaning member.